1. Field of the Invention
The present invention relates generally to linear voltage regulators and specifically to an over-current protection circuit for linear voltage regulators.
2. Background of the Invention
Linear voltage regulators are well known and useful circuits for providing regulation for a load device that draws a changing amount of load current. For example, load regulation maintains a nearly constant output voltage even when the load current varies. In other words, linear voltage regulators provide a relatively constant DC output voltage that is independent of output load current.
FIG. 1 illustrates a conventional linear voltage regulator 2. Linear voltage regulator 2 includes a comparator 4 having a positive terminal coupled to a reference voltage, a negative terminal coupled to a VOUT node, and an output for providing a signal based on the comparison between the reference voltage and the voltage at the VOUT node. An external load 6 is coupled between the VOUT node and ground. External load 6 can be a computer peripheral, such as a keyboard, that has certain current and input voltage requirements.
Linear voltage regulator 2 also includes an output transistor 8. Output transistor 8 includes a drain electrode coupled to VDD, a gate electrode coupled to the output of comparator 4, and a source electrode coupled to the VOUT node.
Unfortunately, the conventional linear voltage regulators suffer from several disadvantages. First, when the load for some reason draws high current through output transistor 8, the linear voltage regulator can be damaged. High current also can cause the following problems: (1) thermal run-away, (2) bondwire failure and (3) electro-migration.
Thermal run-away is an undesirable condition of semi-conductor materials and stems from the negative temperature coefficient of semi-conductor materials. A negative temperature coefficient causes the resistivity of the semi-conductor material to decrease as the temperature of the material increases. As the resistivity decreases, the current through transistors manufactured from semiconductor devices increases. For example, the increase in current in output transistor 8 correspondingly generates heat in transistor 8, thereby increasing the temperature of transistor 8. Accordingly, this vicious cycle continues and causes a xe2x80x9crun-awayxe2x80x9d condition that causes transistor 8 to fail through excessive heat.
High current also can cause bond wire failure in at least two ways. First, the thermal run-away can cause the temperature of circuit elements, such as transistor 8, to reach such a high temperature that the bond wires connected thereto, melt and become fused. Second, high current through an internal VDD and ground short can fuse bond wires together. The fusing of bond wires is undesirable since signals that are designed to have differing voltage levels are corrupted by the voltage of adjacent bond wires, thereby leading to circuit failure.
High current can also cause electro-migration which can cause voids in a metal layer, such as aluminum, thereby increasing the risk of an unwanted open circuit in the current path between the VDD pad and the circuit.
There have been attempts to solve these problems. Unfortunately, these attempts leave much to be desired. Some of these attempts are ineffective in preventing high current conditions. Other attempts shut off the current completely, thereby causing all devices connected down-stream of the current device to lose power. For example, U.S. Pat. No. 5,406,130, entitled, xe2x80x9cCurrent Driver with Shutdown Circuitxe2x80x9d discloses a shutdown circuit to address high current conditions. These prior art approaches involve complex designs and are difficult to integrate into linear regulators, thereby greatly increasing the costs to design and implement the linear voltage regulator circuit.
Furthermore, these prior art approaches fail to address the problems associated with xe2x80x9chot plug-in.xe2x80x9d Many computer peripheral devices are allowed to have a xe2x80x9chot plug-inxe2x80x9d (i.e., the ability to connect and disconnect peripherals to the computer system when the system power is up without first powering down the system). Unfortunately, xe2x80x9chot plug-insxe2x80x9d causes a current surge in the peripheral device that draws current away from the system power. This scenario may cause the system to shut down if the protection circuit does not have a foldback feature. Un-intended system shut down is undesirable because it can cause loss of data, un-intended restart of the operating system, and other failures.
Accordingly, there remains a need for a current protection circuit for linear voltage regulators that overcomes the disadvantages set forth previously.
Accordingly, it is an object of the present invention to provide an over-current protection circuit for linear voltage regulators.
It is a further object of the present invention to provide a current protection circuit for linear voltage regulators that meets the Universal Serial Bus (USB) specification for USB load devices.
It is another object of the present invention to provide a current protection circuit for linear voltage regulators that causes the regulator to provide a fold back current upon detecting certain conditions.
In accordance with the present invention, the foregoing objects are met by providing a current protection circuit that includes a comparator having a first terminal coupled to a reference voltage, a second terminal coupled to an output node, and an output for providing a control signal based on the inputs. The current protection circuit also includes a transistor having a drain electrode coupled to a first predetermined voltage, a gate electrode, and a source electrode coupled to the output node. The transistor includes a current path between the drain electrode and the source electrode for conducting an amount of current that is dependent on the voltage applied to the gate electrode. The current protection circuit further includes a fold-back circuit having a first input coupled to the output node for detecting changes in the output voltage, a second input coupled to the output of the comparator for receiving the control signal, and a third input for selectively changing the voltage applied to the gate electrode based on the control signal and the changes in the output voltage.